1. Field of the Invention
The present invention relates to a conductive composition, a conductive adhesive, and amounting structure of an electronic part using the same.
2. Description of the Related Art
Conventionally, a wire bonding method using solder has been often used when a semiconductor device such as an integrated circuit chip or the like is mounted on the input/output terminal electrodes of a circuit board. However, in recent years, in order to reduce the size of the semiconductor device and to increase the number of connection terminals thereof, the gap between the connection terminals of the semiconductor device is narrowed, which results in gradually making it difficult to mount the semiconductor on the input/output terminal electrodes of the circuit board by the use of a conventional soldering technology.
Accordingly, an attempt has been recently made to reduce the mounting area of the semiconductor device by directly mounting the semiconductor device on the input/output terminal electrodes of the circuit board. Among all, a structure of a flip chip mounting in which the semiconductor device is mounted face down on the circuit board is useful because the structure can electrically connect the semiconductor device to the circuit board by one operation and can produce a large mechanical strength after they are connected to each other.
As the flip chip mounting structure, there are structures in which electrical connections are established by the use of solder, an anisotropic conductive sheet, or a conductive adhesive. FIG. 3 shows a schematic cross sectional view of s flip chip mounting structure in which electrical connections are established by the use of a conductive adhesive. As shown in FIG. 3, in the case where the electrode pad 3 of an IC board 1 is connected to the input/output terminal electrode 7 of a circuit board 4, first, a projecting electrode 2 which is to be an electrical connection point is formed on the electrode pad 3 of the IC board 1 by a wire bonding method or a plating method, and then a conductive adhesive layer 5xe2x80x2 is formed on the projecting electrode 2 by a transfer method.
Next, the semiconductor device is aligned face down such that the projecting electrode 2 is put into contact with the top of the input/output terminal electrode 7 and is then mounted on the circuit board 4. Then, the mounting structure of the semiconductor device is heated to high temperatures to cure the conductive adhesive 5xe2x80x2 to establish the electrical connections.
In this connection, in order to further reinforce the electrical connections, as shown in FIG. 3, the gap between the IC board 1 and the circuit board 4 is sometimes sealed with a sealing resin 6. In this case, a process of sealing and curing the sealing resin 6 is further required.
In this manner, the flip chip mounting structure is fabricated and as a portable digital devices has been reduced in size, a higher mounting density has been strongly required even in the mounting structure of the semiconductor device. Accordingly, even in the flip chip mounting structure described above, the gap between the connection terminals (the gap between the projecting electrodes 2 and the gap between the input/output terminal electrodes 7) has been further narrowed. In the mounting structure having the narrowed gap between the terminals, it is important whether reliability can be maintained or not.
Therefore, the inventors of the present case conducted an accelerated life test for investigating the reliability of a mounting structure which had a DC voltage applied across input/output terminals 7 in the state of high temperature and high humidity. As a result, the inventors found that the following phenomenon occurred: that is, when the above accelerated life test was conducted, water enters the inside of the mounting structure; when the entered water reached the surrounding of the adhesive layer 5xe2x80x2, the conductive particles contained in the conductive adhesive layer 5xe2x80x2 dissolved in the water having reached the surrounding of the adhesive layer 5xe2x80x2; and as a result, the conductive particle in the connection portion was reduced in quantity to gradually increase a connection resistance. Such a phenomenon in which the increasing connection resistance was further increased as the gap between the terminals was narrowed with the increasing mounting density.
Therefore, it is the object of the invention to prevent a connection resistance from being increased by the dissolution of conductive particles.
In order to accomplish the above object, according to the present invention, in short, a pH adjusting agent is added to a conductive adhesive including at least one of resin and rubber, and a conductive particle, whereby when the ambient atmosphere of the conductive adhesive starts to be changed to a pH region in which the conductive particle is easy to dissolve, the pH adjusting agent changes the ambient atmosphere to a stable pH region in which the conductive particle resists dissolving by the neutralization action of the pH adjusting agent, which results in preventing the dissolution of the conductive particle in the surrounding.
It is essential only that the pH adjusting agent in accordance with the present invention can change a pH environment in which the conductive particle is easy to dissolve in the surrounding of the conductive adhesive to a pH environment in which the conductive particle is resistant to dissolving.
The pH adjusting action in accordance with the present invention is required to be selectively produced in a state in which there is a possibility that the conductive particle may dissolve. This is because if the pH adjusting action is produced in a state in which there is no possibility that the conductive particle may dissolve, there is a possibility that the pH adjusting action may have a bad effect on the various characteristics of the conductive adhesive (including electric characteristics).
The state in which there is a possibility that the conductive particle may dissolve, if it is carefully examined, is a state in which water enters the surrounding of the conductive adhesive. In view of this, the pH adjusting agent in accordance with present invention includes a water-soluble substance, whereby the pH adjusting action can be selectively produced in a state in which there is a possibility that the conductive particle may dissolve.
In this case, if the pH adjusting agent includes a substance which is in a solid state in the storage environment and/or actual use environment of the conductive composition, it can be surely prevented that the pH adjusting agent dissolves in the ordinary storage environment and/or actual use environment of the conductive composition.
Further, if the pH adjusting agent is added in a state of powder to the conductive adhesive, it is possible to uniformly arrange the pH adjusting agent in the conductive adhesive. This can uniformly produce the pH adjusting action in the conductive adhesive, which can further surely produce an effect of preventing the conductive composition from dissolving in the surrounding of the conductive adhesive.
Here, it is preferable that the particle size of the pH adjusting agent is smaller than the particle size of the conductive particle. The reason for this will be described in the following: in the case where the particle size of the pH adjusting agent is larger than the particle size of the conductive particle, there is a possibility that the electrical conduction produced by the contact of the conductive particles maybe impaired by the pH adjusting agent having a large particle size; in contrast to this, by making the particle size of the pH adjusting agent smaller than the particle size of the conductive particle, it is possible to prevent the electrical conduction from being impaired by the pH adjusting agent. To be more specific, it is preferable that the particle size of the pH adjusting agent is smaller than 20 xcexcm. In the case where the particle size of the pH adjusting agent is smaller than 20 xcexcm, it is as usual easy to control the amount of transfer of the conductive adhesive to an electrode or the like.
A conductive particle containing Ag, Al, Au, Cu, or Ni is usually used. Therefore, according to the present invention, if the ambient atmosphere of the conductive adhesive containing such a conductive particle starts to change to a pH region in which Ag, Al, Au, Cu, or Ni is easy to dissolve, the neutralization action of the pH adjusting agent can return the pH region to a stable pH region in which Ag, Al, Au, Cu, or Ni is resistant to dissolving, which can prevent Ag, Al, Au, Cu, or Ni from dissolving in the surrounding.
Also, it is preferable that the pH adjusting agent is added to the conductive adhesive at a rate of from 0.1 wt % to 10.0 wt % to the conductive particle. This can produce a pH adjusting action of preventing the dissolution of the conductive particle with a volume specific resistance kept at a sufficiently low value.
Further specifically describing the present invention, a pH adjusting agent having a stable region in an alkaline region is added to the conductive adhesive containing conductive particles (for example, Ag) showing lower solubility in an alkaline region than in an acid region. This constitution produces the following action: that is, even if the ambient atmosphere of the conductive adhesive starts to change to the acid region in which the conductive particle is easy to dissolve, the neutralization action of the pH adjusting agent having a stable region in the alkaline region can return the ambient atmosphere to the stable alkaline region in which the conductive particle is resistant to dissolving. As a result, this can prevent the conductive particle from dissolving in the surrounding.
Also, it is possible to constitute the present invention by a substance having a property opposite to the properties of the above mentioned substances. That is, a pH adjusting agent having a stable region in the acid region is added to the conductive adhesive containing a conductive particle showing lower solubility in the acid region than in the alkaline region (for example, Al), whereby even if the ambient atmosphere of the conductive composition starts to change to the alkaline region in which the conductive particle is easy to dissolve, the neutralization action of the pH adjusting agent having a stable region in the acid region can return the ambient atmosphere of the conductive composition to a stable acid region in which the conductive particle is resistant to dissolving. As a result, this can prevent the conductive particle from dissolving in the surrounding.
In this connection, as the pH adjusting agent having the stable region in the alkaline region, there is an agent including at least one member selected from the group consisting of MgO, Mg(OH)2, 2MgO.SiO2, Al2O3, BaCO3, SnO2, La2O3, Mn3O4, MnO2, CaCO3, Ca(OH)2, BaTiO3, ZnO, CuO, Y2O3, Bi2O3, Pr2O3, Pm2O3, Nd2O3, Y(OH)3, Sc(OH)3, Ce(OH)3, Sm(OH)3, Eu(OH)3, Gd(OH)3, Tb(OH)3, Dy(OH)3, Ho(OH)3, Er(OH)3, Tm(OH)3, Yb(OH)3, Lu(OH)3, ZrO2, Fe(OH)2, Co(OH)2, Ni(OH)2, Cu(OH)2, Cd(OH)2, Zn(OH)2, La(OH)3, Pr(OH)3, Be(OH)2, Al(OH)3, V2O4, Cr(OH)3.nH2O, Ga(OH)2, and PbO. Among these members, MgO, Mg(OH)2, 2MgO SiO2, or Ca(OH)2 has an excellent pH adjusting action as the pH adjusting agent of the conductive adhesive and hardly has a bad effect on the various characteristics necessary for the conductive adhesive (for example, electric characteristics)
Also, as the pH adjusting agent having the stable region in the acid region, there is an agent including at least one member selected from the group consisting of In2O3, TeO2, GeO2, WO3, Gd(OH)3, BeO, Cr2O3, V2O5, Be(OH)2, Al(OH)3, V2O4, Cr(OH)3.nH2O, Ga(OH)2, and PbO.
If the mounting structure of an electronic part is constituted by the conductive adhesive in accordance with the present invention having the above mentioned characteristics, it is possible to provide the mounting structure having a stable reliability.